sulis.fudin.chelsea: Asteroid Belt fly-through/Jupiter flyby (1965)

On January 1, 1801, the first day of the 19th century, astronomer-monk Giuseppe Piazzi discovered a new world between the orbits of Mars and Jupiter. The object, which he named Ceres, was hailed as a new planet - the first discovered since William Herschel found Uranus in 1781. The following year, Heinrich Olbers discovered Pallas in the same region. Olbers told Herschel that he believed Ceres and Pallas to be fragments of a destroyed planet. Herschel suggested that they and any other bodies found between Mars and Jupiter should be considered members of a new category of Solar System bodies, not planets. He suggested that they be called asteroids.

Juno and Vesta were discovered in 1804 and 1807, respectively, but then the discoveries stopped. As a result, Ceres, Pallas, Juno, and Vesta retained a tenuous grip on their planet status. In 1845, however, Astraea became the fifth world found between Jupiter and Mars. It was followed close on by Hebe, Iris, and Flora in 1847, Metis in 1848, Hygeia in 1849, Victoria, Parthenope, and Egeria in 1850, Irene and Eunomia in 1851, and Psyche, Thetis, Melpomene, Fortuna, Massalia, Lutetia, Kalliope, and Thalia in 1852. By the mid-1850s, Herschel's designation for these bodies had won wide acceptance.

As the number of asteroids discovered climbed toward 100, the region in which they orbit became known as the Asteroid Belt. By the centenary of Piazzi's discovery, more than 400 asteroids had been charted. Most follow orbits that keep them always within the Belt, but some - for example, Eros, discovered in 1898 - cross the orbit of Mars and approach Earth. Others - for example, Achilles, found in 1904 - reside at Jupiter's trojan points, 60° ahead or behind the planet along its orbit about the Sun.

By February 1965, when Lockheed Missiles and Space Company submitted the results of a study of robotic Asteroid Belt and Jupiter missions, more than 2700 asteroids were known. Lockheed's study, conducted between July and December 1964 on contract to the Jet Propulsion Laboratory in Pasadena, California, aimed to determine the feasibility of three classes of asteroid missions and to use the asteroid missions as the basis for planning Jupiter flybys.

Lockheed proposed that all of its missions employ a "universal space bus" to which mission-specific components could be added. The company assumed that its spacecraft would rely for electricity on Radioisotope Thermoelectric Generators (RTGs) rather than the solar panels that powered Mariner Venus and Mars flyby spacecraft. RTGs were, it noted, less susceptible to meteoroid damage than the large panels that would be required to generate adequate power beyond Mars. It noted, however, that nuclear fuel for RTGs would be scarce and costly until the 1970s.

The first mission class on Lockheed's list took in Asteroid Belt "minimum flythrough" density missions. These would seek to determine the density of meteoroids in the Asteroid Belt so that engineers could design subsequent Belt-crossing spacecraft with adequate shielding. The first flythrough mission by a 346-pound spin-stabilized spacecraft with six pounds of science instruments might launch as early as 1967, Lockheed estimated. The company acknowledged, however, that no scientific instruments suitable for the mission yet existed; meteoroid detectors designed for use near Earth had low reliability, so were unlikely to function for long enough to reach the Asteroid Belt.

Missions in the second class would be outwardly similar to those in the first. Their primary scientific objective would, however, be to build on the results of the density missions to determine the composition of meteoroids in the Asteroid Belt.

An Atlas rocket with an Agena D upper stage could boost a minimum flythrough mission into a Sun-centered orbit with an aphelion (farthest point from the Sun) at twice Earth's average solar distance, Lockheed estimated, while an Atlas/Centaur would permit a spacecraft to plumb the Asteroid Belt out to 2.25 times Earth's distance. They would thus restrict exploration the the Asteroid Belt's inner edge. An Atlas/Agena D with a High-Energy Kick Stage (HEKS), on the other hand, could boost a flythrough spacecraft to the outermost edge of the Asteroid Belt.

Lockheed's third mission class would see spacecraft fly past Ceres and Vesta at a distance of 1000 kilometers. An Atlas/Agena D/HEKS rocket could launch a 1049-pound spacecraft past either asteroid in any launch opportunity between 1969 and 1980, the company found. With a mind toward ensuring reliability, Lockheed favored launch opportunities that enabled short-duration voyages to Ceres and Vesta. The shortest Ceres mission (360 days) could launch during a 30-day window in 1970, while an opportunity for a 240-day Vesta mission would occur in 1978. For comparison, the longest Ceres mission, launched in 1971, would need 690 days to reach its target, and the longest flight to Vesta (in 1969) would need 550 days.

The Jupiter flybys would build on the asteroid missions. A 1000-pound Jupiter flyby spacecraft could launch in 1971 or 1975 on an Atlas/Centaur/HEKS, but would need about 700 days to reach its target. Lockheed thus advocated the use of more powerful Saturn IB and Titan IIIC rockets. A Titan IIIC/HEKS could boost a 1289-pound flyby spacecraft to Jupiter in only 500 days, while a Saturn IB/Centaur/HEKS combination would permit an even shorter trip time with a more massive spacecraft.

Jupiter orbits the Sun at about 5.2 times the Earth-Sun distance, so a flyby mission to the Solar System's largest planet would have little option but to rely on RTGs, Lockheed found. For the Jupiter flyby, the universal space bus would have plugged into it a seven-foot-diameter dish-shaped high-gain antenna for reliably transmitting data across the enormous distance separating Earth and Jupiter. The "flex-rib" antenna would open like an umbrella near Earth after the spacecraft separated from its booster rocket.

The spacecraft would zip past Jupiter's cloud tops at a distance of about 70,000 kilometers bearing 150 pounds of science instruments. Lockheed noted that some scientists had questioned the need for a camera on the Jupiter flyby; they argued that the planet's "cloud covered surface" would likely be so exotic as to defy interpretation even if it could be glimpsed.

In July 1972, Pioneer 10 became the first spacecraft to enter the Asteroid Belt. During the six-month crossing, the 258-kilogram RTG-powered spacecraft detected far fewer dust particles and meteoroids than expected. It departed the Asteroid Belt undamaged in February 1973, and flew past Jupiter at a distance of 130,000 kilometers on December 5, 1973 (top image above). Having braved the imagined perils of the Asteroid Belt, the intrepid robot explorer suffered damage in Jupiter's radiation belts, scrubbing a planned television survey of Io, the planet's innermost large moon.

The Jupiter-bound Galileo spacecraft was the fifth spacecraft to enter the Asteroid Belt. On October 29, 1991, it became the first to fly past an asteroid (Gaspra, the 951st asteroid discovered, at a distance of 1604 kilometers). Galileo zipped past Ida, the 243rd asteroid discovered, at a distance of 2410 kilometers on August 28, 1993. Ida was found to have a kilometer-wide moon, which scientists named Dactyl. By then, controllers on Earth had largely abandoned their efforts to unfurl Galileo's umbrella-like high-gain antenna, several ribs of which had jammed during deployment on April 11, 1991.

The Dawn spacecraft left Earth on a Delta II 7925H rocket on September 27, 2007. The solar-electric propulsion spacecraft performed a gravity-assist flyby of Mars on February 17, 2009, and arrived in orbit around grooved, cratered Vesta on July 16, 2011 (images below), making it the first spacecraft to orbit a Main Belt asteroid. Dawn is scheduled to depart 530-kilometer-diameter Vesta in mid-2012 bound for Ceres. If all goes as planned, it will orbit spherical, 950-kilometer-diameter Ceres - the only dwarf planet inside the orbit of Neptune - in February 2015.